The mechanisms of cardiac defibrillation are subject of major interest in the treatment of human heart disorders. Significant progress in the understanding of defibrillatory action has been made recently through advanced high-resolution mapping studies. The research focus s has now advanced from the traditional empirical approach to the mechanistic interactions of shocks and wavefront propagation. Recent progress in optical imaging techniques has enabled such studies to detailed spatial information of the transmembrane potential (Vm) distribution around the entire heart throughout the defibrillation episode, providing unobstructed panoramic views of the epicardial wavefronts. Similarly, experimental results using optical imaging have promoted the concept of cardiac bidomain from theoretical considerations to a valid representation of myocardium, thus generating new hypotheses in defibrillation mechanism and prompting the necessity to re-evaluate traditional ones. The major goal of this proposal is to study basic defibrillation mechanisms for the perspective of cardiac bidomain. Furthermore, this project is the first attempt to use dynamic high resolution Vm images interactively in defibrillation by including the imaging result as a feedback component in a real-time, intelligent multi-electrode shock delivery system. The interactive shock system allows the test of important hypotheses pertaining to the mechanism of defibrillation, which historically have been examined empirically without such capability. We will investigate 91) the interaction of direct activation sites with the tissue status during the shock; (2) the optimal timing for the shock delivery during different stages of the formation on f a reentry pattern; and (3) the optimal timing and sites to deliver the shock during the interaction of multiple spiral recenter patterns. Bu7 combining various predictions and hypotheses related to the concept of bidomain continuum for sycytial cardiac tissue, it is expected that the results from this proposal will offer new insights of the fibrillation mechanisms based on spatia-temporal wavefront interactions as predicted by the cardiac bidomain, and could provide practical guidelines for the future, improvement of arrhythmia intervention.